Some programs are broadcast in a “letterbox” or “widescreen” format. This format converts a program (e.g., motion picture movie) that has been engineered for the big screen to fit on a conventional television display. Conventional TVs have an aspect ratio of 4:3 (measured as a ratio of width to height). The letterbox format shrinks the program width to fit on the TV screen.
Unfortunately, when you shrink the width, the height is also compacted or else the image is distorted. Shrinking the program vertically leaves gaps at the top and bottom of the screen, which are simply filled in with black. When played, movies in letterbox format use a limited number of lines on the TV, leaving black bands at the top and bottom of the TV screen. These black bands are familiar to viewers who have watched a VHS or DVD movie that has not been reformatted for the TV. The thickness of the bands depends on the width of the movie. Most movies today are 1.85 times as wide as tall, or a ratio of 1.85:1. Some movies are wider at a ratio of 2.35:1, such as the motion picture movie Star Wars.
Newer widescreen television sets are designed with an aspect ratio of 16:9. Widescreen television sets are designed to stretch the picture 33% horizontally and 33% vertically, enlarging the total picture by 78%. As a result, widescreen TVs are able to expand letterboxed movies to better utilize the screen area.
Some movies released on DVD (digital video disk or digital versatile disk) are mastered in an “anamorphic” format where the program is pre-stretched vertically by a ratio of 4/3 (four-thirds). Anamorphic DVDs use 33% more of the storage area for the image, by stretching the image vertically by 33% and thereby increasing the number of lines. When anamorphic DVDs are played on widescreen TVs, the TVs operate in a mode that expands the program 33% in the horizontal direction, without vertical expansion since the vertical stretch is inherent in the anamorphic format. As a result, the program fits the whole screen, eliminating the black banding on the top and bottom of the screen. When anamorphic DVDs are played on standard 4:3 ratio TVs, the DVD players are designed to squeeze the image back down to normal. They use weighted averages to combine lines, scaling the image back down by 33%.